Production of aliphatic acids



PRODUCTION OF ALIPHATIC ACIDS No Drawing. Application October 9, 1953,

Serial No. 385,272

Claims priority, application Great Britain October 15, 1952 6 Claims. ((31.260-533) The present invention relates to the production of aliphatic acids, particularly acetic acid.

It has been found in accordance with our invention that in the oxidation of paraflins in. the liquid phase by means of oxygen at elevated temperature andpressure, the products include volatile non-acidic oxidation products', such as ketones and esters, lower aliphatic acids from formic acid upwards, and high boiling residues which remain after removal of the acids by distillation. The high-boiling residues are complex materials which have little direct usefulness, or which require further treatment for conversion to useful products.

It has also been found in accordance with our invention that in such oxidation of paraflins of a given number of carbon atoms, the highest proportion of residues are obtained from the member having a straight carbon chain, and that the proportion of residues to acids produced is reduced by the presence of branched side chains in the molecule.

This may be illustrated by reference to the oxidation of pure hydrocarbons containing 6 carbon atoms, namely n-hexane and the two methyl pentanes. These were oxidised under identical conditions with air at 140160 C. and 200 lbs/sq. in. pressure until the oxygen contained in the air feed was no longer completely absorbed. The products were then worked up by distillation to separate the following successive fractions: volatile nonacidic products, volatile aliphatic acids of one to four carbon atoms together with water, and high boiling residues. The amounts of the residues obtained, expressed as parts by weight based on 100 parts of anhydrous acids of one to four carbon atoms recovered were then as follows:

Hydrocarbon oxidised: Residues n-Hexane parts 43.6 Z-methylpentane do 21.7 3-methylpentane do 13.5

It is an object of our invention to obtain lower aliphatic acids by the oxidation of parafiinic hydrocarbon, while at the same time obtaining as low a proportion as possible of high boiling residues in the oxidation products.

tates Patent Patented July 23, 1957 The invention accordingly comprises the novel processes and steps of processes, specific embodiments of which are described hereinafter by way of example and in accordance with which I now prefer to practice the invention.

Accordingly, the present invention is for a process for the production of acetic acid which comprises oxidising, in the liquid phase with a gas consisting of or comprising molecular oxygen, a parafiinic hydrocarbon fraction containing hydrocarbons of 4-8 carbon atoms at least 40% by weight of said fraction consisting of hydrocarbons of 6-8 carbon atoms, wherein at least 40% by weight of the paraffins of 6-8 carbon atoms of said fraction consists of parafiins having one or more methyl branch chains and wherein said fraction boils at a temperature not exceeding 100 C. Desirably the fraction boils within the range about 15 to about C.

By the expression methyl branch chain is meant a parafiin bearing a methyl group in a non-terminal position in ,a chain.

Suitable feedstocks which may be used are straight run petroleum fractions containing a high proportion of branched molecules; the products from isomerisation of predominantly straight chain parafiins by aluminium chloride or from other isomerisation processes known to the art; the products obtained by the alkylation of olefines with isoparatlins; or alternatively the product obtained by hydrogenation of the mono-olefines produced by polymerisation of lower olefines such as propylene and isobutene. For optimum economic operation of the process it is of course necessary to balance the desirable properties of the fedstock against the cost of its production.

An example of a cheap commercially available paraffinic hydrocarbon feedstock suitable for the process of the invention is a straight run gasolene fraction from a Middle East petroleum boiling between about 15 and 95 C.

Batchwise oxidation of this material at ISO-160 C. under conditions comparable with those used for the oxi dation of n-hexane, Z-methylpentane and B-methylpentane as described above yielded residues amounting to 26.4 parts by weight based on parts of anhydrous acids of one to four carbon atoms recovered.

By way of comparison in the oxidation of a fraction of the same origin but of higher upper boiling point, for example of up to C., appreciable corrosion of stainless steel equipment occurs which is not the case with the fraction boiling from 15 95 C. and further this fraction boiling up to 150 C. yields residues amounting to about 100 parts per 100 parts by weight of acids of one to four carbon atoms recovered.

As oxidising gas there may be used any gas containing molecular oxygen, whether in the form of air or of mixtures poorer or richer in oxygen than is air; part of the molecular oxygen may, if desired, be in the form of ozone. The use of high pressures in the oxidation may be necessary in order to maintain the reactants in the liquid phase.

The temperature of oxidation desirably should be sufficiently high to provide an economically attractive rate of oxidation whilst producing a low level of peroxidic compounds, whilst of course the temperature should not be so high that excessive oxidation occurs of the hydrocarbons or of their primary oxidation products to oxldes of carbon and water. Temperatures in the range/about l30-200 C- have been found suitable, although higher or lower temperatures may be used if so desired.

they may be dispensed with. Similarly, while materials such as solvents for the hydrocarbons, peroxides, initiators and the like, may be added if desired, these are not necessary for the practice of the invention and may impair the economy of the process.

The oxidation maybe carried out batchwise or continuously. It has been found that the reaction mixture is normally a homogeneous liquid at operating temperature, but that on cooling two liquid layers separate. Of these the upper layer is mainly unchanged hydrocarbon, while the lower layer contains the bulk of the acidic products and water. It is therefore an important feature of the process to operate by removing continuously or intermittently a part of the reaction mixture as a homogeneous liquid, to cool this to a temperature below about 80 C. and preferably to as near room temperature as may be convenient, to separate the liquid layers, to return all the upper layer to the reactor and to remove all or part of the lower layer as product. It is a further feature of our process to replace the part of the product thus withdrawn with fresh hydrocarbon so as to maintain a constant volume of liquid in the reactor system, thus effecting continuous operation of the oxidation process.

The oxidation may be carried out in a variety of types of apparatus provided only that the reaction mixture is maintained substantially in the liquid phase, and that adequate contacting is effected between the liquid reaction mixture and the oxidising gas. Thus, the oxida tion may be carried out in a pressure reactor fitted with a mechanical stirrer for dispersion of the gas throughout the liquid. Alternatively, the reactor may be in the form of a vertical tower with the feed of oxidising gas at the base or at other points up the'tower, in which the necessary agitation is effected by the gas itself; in this case it is advantageous to subdivide the gas feed by mechanical means to obtain a fine dispersion at the point of entry and throughout the reaction zone. Alternatively the oxidising gas may be fed into a stream of liquid moving with high velocity in a circulatory system, such as a coil reactor with forced circulation. I

For construction of the reactor it is preferred to employ materials with sufficient degree of resistance to organic acids, although the corrosive properties of the oxidation mixture are less than those of the most highly corrosive constituents. Thus we have found a stainless steel containing 18% chromium, 8.5% nickel, 0.6% titanium (Firth-Vickers FDP) to be a satisfactory material of construction, although certain of the minor constituents of the mixture such as formic acid, are by themselves corrosive towards this steel.

The following example is given to illustrate the process of the present invention.

Example The oxidation apparatus consisted of a stainless steel reactor fitted with a stirrer, a bottom air inlet, a line for removing waste gas, connections for the addition and withdrawal of liquid and means for measuring and com trolling the temperature. Provision was made to withdraw continuously a part of the liquid products. through 4 ing an initial boiling point (A. S. T. M.) of 31.5 C. and a final boiling point (A. S. T. M.) of 96.5 C. having the composition:

Paraffins of 6-8 carbon atoms having methyl branch chains, percent w./w. of traction Parafiins of tion, 68 carbon atoms, percent W./W. of traction n-Butane Oyclopentane 2:2-Dlmethylbutan n-Hexane Benzene Cyclohexane 2:3-D'tmethylpentane Z-Methylhexane 1:3-Dirnethylcyclopentane (eis isomer) 3-Methylhexane No oxidation catalyst was added. The temperature was raised to 160 C. and the pressure to 300 lbs./sq. in. The air feed was started at a low rate during the induction period ofabout 30 minutes, after which it is increased to 8-10 litres/minute, and this air rate was maintained during the experiment with virtually complete absorption of oxygen at a reaction temperature of 160 C.

Circulation of the liquid reaction mixture through the cooler and separator was started at a rate of about 0.6 litres/hour soon after oxygen absorption began, and as soon as a lower aqueous acid layer began to appear in the separator it was withdrawn continuously as product and replaced with fresh hydrocarbon feed, sufiicient to maintain a constant volume of liquid in the system, at an' average rate of 0.26 litres/hour.

The oxidation was continued for a total of 37 hours, after which the product was distilled to recover the separate constituents in successive fractions, namely volatile non-acidic products, lower aliphatic acids together with water, and high boiling residues. The wet aliphatic acid fraction was subsquently dehydrated and the anhydrous acids rcovered' in successive fractions. The yields obtained, on the basis of parts of hydrocarbon consumed in the oxidation on leaving the system in the exit gas, were as follows:

. Parts Vol'atilenon-acidic products 33.8 Water 16.8

Formic acid 4.7

Acetic acid 28.5

Propionicacid 6.5

Butyric acid 1.7

Total lower aliphatic acids; 41.4

High boiling residues 15.1

The production of residues, on the basis of parts by weight corresponding to 100 parts of anhydrous C1-C4 aliphatic acids recovered, was, therefore, 36.5 parts.

By way of comparison the'process described above was repeated: employing in the place of the paraffin fraction used a paraffinic distillation fraction from a Middle East crude petroleum having an initial boiling point (A. S. T-

M.) of 31.5 C. and a final boiling point (A. S. T. M.) of 150 C.

The yields obtained on the basis of 100 parts of hydrocarbon consumed in the oxidation on leaving the system in the exit gas were as follows:

The production of residues, on the basis of parts by weight corresponding to 100 parts of anhydrous C1-C4 aliphatic acids recovered, was therefore 99.6.

We claim:

1. A process for the production of acetic acid which comprises heating in the liquid phase to a temperature in the range of about 130-200 C., a paraflinic hydrocarbon fraction with molecular oxygen, said fraction containing hydrocarbons of four to eight carbon atoms, at least 40% by weight of said fraction consisting of hydrocarbons of six to eight carbon atoms, said last named fraction having at least 40% of paraflins having at least one methyl group in a non-terminal position, the upper boiling point of said fraction not exceeding 100 C., and oxidizing said hydrocarbon fraction to give C1 to C4 aliphatic acids as a principal oxidation product, of which acetic acid is the major constituent.

2. A process as in claim 1 wherein said fraction boils within the range about l5-95 C.

3. A process as in claim 1 wherein said fraction is a straight run gasolene fraction from a Middle East petroleum boiling in the range 15 -95 C.

4. A process for the production of acetic acid which comprises heating in the liquid phase to a temperature in the range of about 130-200" C. with molecular oxygen, at paraflinic hydrocarbon fraction containing hydrocarbons of 4-8 carbon atoms, at least 40% by weight of said fraction consisting of hydrocarbons of 6-8 carbon atoms wherein at least 40% by weight of the paraflins of 6-8 carbon atoms of said fraction consists of paraflins having at least one methyl branch chain in a non-terminal position, said fraction being of boiling range not exceeding 100 C. and oxidising said hydrocarbon fraction to give C1 to C4 aliphatic acids as a principal oxidation product, of which acetic acid is the major constituent, during the course of which oxidation a part of the oxidation reaction mixture is withdrawn from the oxidation zone, cooled to a temperature below C. and allowed to separate into two layers, and the upper layer returned to the reaction zone, all or part of the lower layer being removed as product.

5. A process as claimed in claim 4 wherein the amount of the lower layer Withdrawn as product is replaced by an equal amount of the paraflinic hydrocarbon fraction.

6. A process which comprises heating a distillate of straight-chain and branch-chain hydrocarbons having 4-8 carbon atoms with molecular oxygen at about 160 C. and at about 300 lbs/sq. in. pressure, at least 40% by weight of said hydrocarbons having 6 to 8 carbon atoms and at least 40% by weight of said hydrocarbons having 6 to 8 carbon atoms having at least one methyl branch chain in a non-terminal position, the constituents of said distillate boiling between 30 and C., and oxidising said hydrocarbons to give C1 to C4 aliphatic acids as a principal oxidation product, of which acetic acid is the major constituent.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR THE PRODUCTION OF ACETIC ACID WHICH COMPRISES HEATING IN THE LIQUID PHASE TO A TEMPERATURE IN THE RANGE OF ABOUT 130-200*C., A PARAFFINIC HYDROCARBON FRACTION WITH MOLECULAR OXYGEN, SAID FRACTION CONTAINING HYDROCARBONS OF FOUR TO EIGHT CARBON ATOMS, AT LEAST 40% BY WEIGHT OF SAID FRACTION CONSISTING OF HYDROCARBONS OF SIX TO EIGHT CARBON ATOMS, SAID LAST NAMED FRACTION HAVING AT LEAST 40% OF PARAFFINS HAVAING AT LEAST ONE METHYL GROUP IN A NON-TERMINAL POSITON, THE UPPER BOILING POINT OF SAID FRACTION NOT EXCEEDING 100*C., AND OXIDIZING SAID HYDROCARBON FRACTION TO GIVE C1 TO C4 ALIPHATIC ACIDS AS A PRINCIPAL OXIDATION PRODUCT, OF WHICH ACETIC ACID IS THE MAJOR CONSTITUENT. 